1. Field of the Invention
The present invention relates to a blood-pressure monitoring apparatus which monitors the blood pressure of a living subject without causing the subject to feel serious discomfort.
2. Related Art Statement
There is known a blood-pressure ("BP") monitoring device which monitors the blood pressure of a living subject such as a patient. The BP monitoring device includes a BP measuring device which includes an inflatable cuff adapted to be wrapped around a body portion (e.g., upper arm) of a patient and which measures one or more BP values of the patient by changing the air pressure in the inflatable cuff wrapped around the upper arm of the patient. The BP monitoring device periodically starts a BP measurement of the BP measuring device at a predetermined period. However, this BP monitoring device has the problem that if the BP measuring device starts a BP measurement at a shorter period, for improving the reliability of monitoring of the BP monitoring device, the inflatable cuff more frequently presses the upper arm of the patient and causes the patient to feel serious discomfort.
In this situation, there has been proposed another BP monitoring device which includes a pulse-wave detecting device which detects a pulse wave which is produced in an inflatable cuff when the cuff presses an upper arm of a patient; a pressure control device which repeatedly increases the pressing pressure of the inflatable cuff (hereinafter, referred to as the "cuff pressure") up to a predetermined pressure value lower than a mean BP value of the patient, with a predetermined pause being inserted between each pair of repeated cuff-pressure increasing operations; a change-rate calculating means or circuit which calculates the rate of change of respective amplitudes of heartbeat-synchronous pulses of the pulse wave detected by the pulse-wave detecting device when the cuff pressure is increased to the predetermined pressure value by the pressure control device, with respect to the cuff pressure increased by the pressure control device; and an abnormality judging means or circuit which judges, based on the calculated rate of change, whether the patient has an abnormal BP value. This BP monitoring device is disclosed in, e.g., U.S. Pat. No. 5,649,536. Since the BP monitoring device changes the cuff pressure in a low pressure range from the atmospheric pressure to the predetermined pressure value, and monitors the BP of the patient based on the rate of change of the pulse amplitudes obtained during the changing of the cuff pressure, it does not cause the patient to feel serious discomfort.
However, even in the above-indicated second BP monitoring device, the inflatable cuff repeatedly presses the upper arm of the patient, though the pressing pressure of the cuff, i.e., the cuff pressure is not increased to a high pressure value. If the frequency of pressing of the cuff is increased to improve the reliability of the BP monitoring device, the patient may feel serious discomfort.
Meanwhile, there has also been proposed yet another BP monitoring device which iteratively calculates an index value indicative of a condition of a circulatory system of a patient and monitors a change of the BP of the patient based on a change of each of the calculated index values. The index value may be one which is indicative of a condition of circulation of blood in the patient and which can be non-invasively obtained without pressing any body portions of the patient. For example, the index value may be a piece of pulse-wave-propagation ("PWP") relating information, such as a PWP velocity or a PWP time, that relates to propagation of a pulse wave through an arterial vessel of the patient; a piece of pulse-relating information, such as a pulse rate (i.e., heart rate) or a pulse period; or a piece of blood-flow-amount-relating information, such as a peripheral-pulse-wave area, that relates to an amount of flow of blood through a peripheral body portion of the patient. The third BP monitoring device is disclosed in, e.g., U.S. Pat. No. 5,776,071. The disclosed BP monitoring device iteratively calculates, as the index value, a value relating to each of heartbeat-synchronous pulses of a volumetric pulse wave obtained from a patient, and monitors the BP of the patient based on a change of each calculated value. When the third BP monitoring device finds a significant change of each calculated value (i.e., each index value), it starts a BP measurement of a BP measuring device using an inflatable cuff, so as to obtain one or more reliable BP values of the patient. However, if too low a reference value is employed to find a change of each index value, for the purpose of improving the reliability of the BP monitoring device, the cuff would too frequently press the upper arm of the patient and would cause the patient to feel serious discomfort.